“Sometimes it’s high – like on
foggy mornings or those ‘sticky’ summer days familiar to
people living in the north of the North Island – and
sometimes it’s low, like when the nor’wester blows in
Canterbury.

“Humidity is always there; there’s
always some water vapour suspended in the air.”

It’s the combination of extreme humidity and high
temperatures that can really play havoc with our wellbeing,
says Dr Revell. That’s why summer is the time to beware of
humidity’s more unpleasant effects.

“The higher
the air’s temperature, the more water vapour it is capable
of holding. So on a hot summer day when the humidity is high
– or in the tropics at any time of the year – there’s
a lot of water vapour out there interfering with the way our
body functions.

“And a hot day with very low
humidity feels different – but can be just as
debilitating.”

The term ‘relative humidity’ is
the actual amount of water vapour present in the air divided
by the maximum amount the air could hold at its current
temperature, usually expressed as a percentage.

“So when relative humidity gets up into the 80s and
90s, you know the air is nearly as full as it can be with
moisture. On the other hand, it can sometimes fall as low as
20 or 30 per cent,” explains Dr Revell.

Natural
air conditioning

To understand how extreme humidity
and high temperatures affect us, we need a quick explanation
of how our body’s natural cooling system works:

When our body temperature rises, we sweat. The water on
our skin then evaporates into the air and, “if you recall
your year 11 science,” explains Dr Revell, “you’ll
know the process of evaporation draws energy from the air,
reducing its temperature.

“That’s how our body
attempts to keep us cool. Evaporating sweat creates a thin
layer of cooled air close to our skin.

“But when
humidity is high and the air is already laden with moisture,
our sweat can’t evaporate as quickly and the cooling
effect is reduced.”

The result? That feeling like
we’re covered in a warm, damp blanket.

When
extremely high temperatures and humidity strike, the result
can be dramatic – and even dangerous.

“As our
body works harder and harder to cool us down,” explains Dr
Revell, “precious water is diverted from our brain and
internal organs, slowing them down and making them work
harder. Our blood thickens, making it harder for our heart
to pump it around our body.”

Hyperthermia, or
‘heatstroke’, is the ultimate result of dehydration and
an overheated body. Symptoms are unpleasant and potentially
life-threatening – particularly among the elderly or
unwell.

And as if that’s not enough, high
temperatures and humidity cause irritants like mould spores
and dust mites to proliferate – creating misery for
sufferers of allergies and asthma.

Beating the
blues

When conditions turn hot and muggy, there are
some basic precautions you can take to lessen the
danger.

“So the key is to reduce activity, pace yourself, and
drink plenty of water to replenish your body’s cooling
system.”

In New Zealand, prolonged periods of
extremely high temperatures and humidity are mercifully
rare. For most, bad hair is the most alarming effect we’re
likely to experience.

‘The frizz’ happens when
abundant moisture in the air begins to break down temporary
hydrogen bonds in our hair. The process allows more
permanent sulphur bonds to prevail.

So in fact, high
humidity allows our hair’s natural tendencies to show
through: if it’s naturally inclined to curl or wave, high
humidity will bring out the frizz; if it’s inclined to be
straight, a limp coiffure will greet us in the mirror on a
muggy morning.

“Interestingly, human hair is so
sensitive to humidity changes that it was once used as the
active component in meteorological instruments called
hygrometers, which graph changes in humidity over time,”
says Dr Revell.

The dry horrors

In the east
of both islands, and in inland Canterbury and Otago, a
combination of high temperatures and low humidity is a much
more common occurrence.

“Here the effect is quite
different,” says Dr Revell. “Sweat evaporates off our
skin much faster because the air has plenty of capacity to
take up more moisture. As a result, the cooling effect is
more pronounced. In fact, it’s possible you’ll feel
quite chilly, even though the temperature may be in the
twenties.”

But dehydration is just as big a
danger, he adds. “Cantabrians know to drink plenty of
water when the nor’wester blows and the humidity drops,
because moisture is being sucked up at such a great
rate.”

Dry lips, a dry throat and irritated skin
and eyes are common problems on Canterbury’s low-humidity
days. What’s more, dried-out mucus membranes in our nose
and throat can ease the passage of bacteria and irritants
into our system – prolonging the misery for some.

Static electricity build-up is another side-effect of
very dry air. “Take care when reaching for the light
switch or car door handle on low-humidity days,” says Dr
Revell. “A short sharp zap could be in store.”

Of course it’s not all gloom and doom. Warm, humid
weather encourages luxuriant growth in the garden, while low
humidity is great for getting the washing dry and enjoying a
good night’s sleep.

“We just need to understand
humidity’s effects and take some basic precautions,”
says Dr Revell. “Then we can get on with enjoying
summer.”

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